1. Field of the Invention
The present invention relates to a battery protection circuit for protecting a plurality of batteries connected in series, and to a battery device.
2. Description of the Related Art
Portable devices such as a laptop computer and a cellular phone, in some cases, include a battery device which includes a plurality of batteries connected in series and first and second battery protection circuits. The first battery protection circuit controls charge/discharge of the battery to protect the battery. Should the first battery protection circuit not operate when it is supposed to operate, in order to prevent the battery from igniting due to its overdischarge, the second battery protection circuit stops a function of the battery device. The second battery protection circuit performs the function of ultimately protecting the battery, and thus the battery device may not be used when the second battery protection circuit operates (for example, see JP 2000-295777 A).
More specifically, as illustrated in FIG. 4, a second battery protection circuit 10 monitors voltages of batteries BAT1 to BAT4, and cuts off a fuse 20 provided in a charge/discharge path when at least any one of the voltages is equal to or more than a predetermined voltage. As a result, the charge/discharge path is interrupted, and the battery device is stopped.
Here, as illustrated in FIG. 4, the second battery protection circuit 10 includes a parasitic diode which is formed between a P-well 12a connected to an intermediate terminal VC2 and an N-substrate. In addition, the second battery protection circuit 10 includes a parasitic bipolar transistor 12 in which a base is the N-substrate, an emitter is the P-well 12a, and a collector is a P-well 12b. Moreover, it is assumed that a positive electrode terminal of the battery BAT1 at the uppermost stage is a power supply terminal VDD, a negative electrode terminal of the battery BAT4 at the lowermost stage is a ground terminal VSS, and terminals of the batteries BAT2 and BAT3 are intermediate terminals VC1 to VC3.
Then, for example, when only the intermediate terminal VC2 and the ground terminal VSS are connected to the batteries, a forward current flows from the intermediate terminal VC2 to the power supply terminal VDD via the parasitic bipolar diode, and the parasitic bipolar transistor 12 operates in response to the forward current (base current), whereby a logical circuit 11 malfunctions in some cases. Due to this malfunction, in some cases, the second battery protection circuit 10 outputs a signal for making the battery device unusable.
Accordingly, limitations should be placed on an order of connecting the respective terminals of the second battery protection circuit 10 to the respective batteries, whereby a manufacturing process for the battery device is complicated to reduce a yield, leading to an increase in manufacturing cost for the battery device.